An outline of the progression and mechanism of ship hull fouling in sea water, with discussions concerning types of organisms involved ad corrosion effects, can be found in corrosion Handbook, W. F. Clapp, John Wiley and Sons, Inc., New York, 1948, and World Atlas of Coastal Biological Fouling -- Part 1, E. J. Pastula, Naval Oceanographic Office, Washington, D. C., 1R No. 70-51, 1970. Thus, marine growth on metal surfaces freshly immersed in sea water customarily follows a definite pattern. Bacteria dominate the new surface for about the first 3 days and are succeeded by algae slimes between about the third and seventh days. The next 2 weeks see protozoans taking over and, by the third week, barnacles are becoming firmly established. Tunicates appear between about the tenth and sixteenth weeks after which grasses predominate until about the sixth month. Thereafter, mussels are seen to have taken a firm hold. Unfortunately, these fouling organisms are cumulative such that buildup on the hull of a ship is quite rapid.
One inch depth of fouling adversely affects the streamlining of a ship's hull, weighs much, ruins skin smoothness, and greatly alters corrosion processes that are taking place on structural members and component parts. In addition, these organism may cuase plugging problems in condensers, pipelines, and valves exposed to contact with sea water. Finally, this marine growth can exert a significant effect on friction coefficient and pumping efficiencies for sea water handling systems. Such circumstances have the very practical effect of requiring commercial shippers to reckon on increases in fuel consumption depending upon the length of time since the last haulout for hull cleaning. For example, a 1% increase in power is typically required for each 4 days after haulout. This results in the need for a 22% increase after about 3 months and more than a 45% increase in 6 months.
Wooden vessesl are also subject to attack by Toredo navalis and other organism, e.g., Limnoria, which bore into wood in populations of up to 5000/ft..sup.2. The Teredo worm commonly lives for about 2 1/2 months, boring a hole about 6 inches in length and 1/4 inch in diameter. These organism are present in essentially all areas of the oceans.
The two areas of a ship's hull which require special coatings are the bottom and the boat-topping area. The boat-topping area, being intermittently exposed to both air and sea water, present and especially difficult surface to protect from the elements and marine organisms.
For use as anti-fouling coatings on ship bottoms, copper, mercury, and/or tin compounds are frequently incorporated into binders which are somewhat water sensitive. Gradual breakdown of the binder occurs which affords a sustained release of the toxic metal compounds. A tin-based toxic biocide has been recommended for aluminum hulls and outboard lower units. Copper will destroy aluminum by galvanic action and the contact of aluminum with the copper agent will, as with other metals less noble than copper, render the copper ineffective as an anti-foulant. Therefore, a copper-containing coating must be insulated from the aluminum (or steel) hull by an anti-corrosion coating to prevent migration of copper ions thereto and causing oxidation of the metal hull.
The binders in ship bottom paints customarily comprise either a porous type or one having a surface which continually washes away. The former, denominated the "hard" type, usually has either a vinyl or epoxy vehicle chemistry. The latter or "soft" type normally has a pine tar or gum base and is somewhat less hazardous to apply.
In general, the binders for boat-topping paints are designed to provide a high level of resistance to both salt water and weather. Phenolic resin-tung oil and vinyl resin combinations have commonly beeen utilized.
A glass additive containing a biocidal agent can offer several advantages over the present commercially-marketed anti-fouling coatings:
(1) the glass composition can be tailored such that the metal ion constituent of the coating will not leach or diffuse into the aquatic environment at an excessive rate;
(2) the coating can be designed to remain effective for a longer period of time than the normal 3-18 month life span of the conventional coatings;
(3) the overall cost of the coating can be cheaper but with the same degree of efficiency; and
(4) the glass-containing coating will be less polluting since the conventional coatings leach out relatively rapidly in sea water, thereby contributing heavy loads of metal in harbors.